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spray foam insulation for underside of roof
Last Post 01 Jun 2016 06:26 PM by Dana1. 201 Replies. |
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Dana1
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| 13 Apr 2009 02:40 PM |
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Posted By jcwells on 04/12/2009 4:11 PM
Thanks for the responses. I’ve since had a quote on Icynene (or equivalent) for the full attic sheeting and it’s not too bad. Given the extra freedom it gives us for duct placement and flexibility with electrical changes down the road, I am strongly leaning toward this for the attic. I also like the idea that we can get the house insulated now, rather than wait until electrical and drywall is completed. As you can imagine the house warms very fast with the attic open to the house.
I am still uncertain on the walls however. Remember, I have no stud cavities, just 1X3 strapping on top of the block. I don’t think ¾ inch of ½ pound foam would do that much good in those cavities (difficult to spray too), whereas ¾ inch iso board over the complete wall with no thermal breaks, gives me R5 plus a ¾ inch air space over the full wall. I realize that the foil faced iso is a vapor barrier, but from what I have read the dew point is within the board so the foil does not create a condensation problem. I like the idea that it does, however, restrict vapor entry into the house even though technically it would be better on the outside (not an option). Still thinking on that one.
Further, I’m surprised that the consensus is I will have mold problems in the unoccupied house. With nothing to trigger condensation and a careful selection of finishes, I would not have thought this would be a problem. Can anyone share any first hand experiences of homes left unventilated for months in a zone 1 climate?
Thanks,
John
You're right that a skim-coat of half-pound foam isn't going to buy you much, but you might look at going with unfaced 1" fiberglass-reinforced iso sheathing with a permeability of about 3 perms, or 1.5" EPS (~1 perm?) rather than foil-faced iso, which will deliver the same R5 as 3/4" foil-face iso . (I think the higher perm reinforced iso is available in 1.5" R7 as well?) Those furring strips may still have mold/rot issues, unless they're rated for ground contact (pressure-treated, etc.) As long as the 3/4" cavity between the insulating wall (the iso) and the block is preserved, even foil faced iso could work, but without some air-movement through the cavity is allowed it may still need at least some inward-drying capacity. Semi-permeable or low-perm insulation is still safer. XPS can be used if you stick with 3/4" (~R3.8) or less, but at 1" & up it's permeability is too low. (Me, I'd go with 2" of unfaced iso or 3" of EPS bringing it up to R12, and give up the square foot or two of room area for each, if it doesn't mess with window & door framing much to get there.) In the humid south you DON'T want ventilation when unoccupied. Pulling warm high-humidity outside air into the cooler (earth-coupled/insulated) interior raises the RH as it cools, enhancing the mold growth. Mechanical dehumidifcation is the only safe answer. Read up on it- spend some time at it until your comfortable before deciding on which way to go on the walls. http://www.buildingscienceconsulting.com/designsthatwork/hothumid/bestpractices.htm http://www.buildingscienceconsulting.com/designsthatwork/hothumid/profiles/orlando.htm This one may be similar enough to YOUR actual construction type: http://www.buildingscienceconsulting.com/designsthatwork/hothumid/profiles/maitland.htm
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jcwells
New Member
Posts:4
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| 13 Apr 2009 06:14 PM |
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OK, this has been very helpful. It’s sinking in that the wall insulation must allow the vapor that migrates through the block to dry to the inside. I will settle on one of the suggested alternatives and ditch the foil faced iso for the walls.
Also, I will plan to add a dehumidifier that will run independent of the A/C system. Perhaps one of the site modified consumer dehumidifiers as suggested on buildingscienceconsulting.com.
Thanks,
John
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LisaBasis
New Member
Posts:3
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| 17 Apr 2009 12:21 PM |
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Can you tell me the name of the synthetic furring channels that were mentioned and where they are available? |
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Alton
Veteran Member
Posts:2164
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| 17 Apr 2009 01:22 PM |
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LisaBasis,
I saw the following listed product at the 2007 International Builders' Show in Orlando:
BattenUP Battens by Battens Plus, Inc. 530-620-5287, www.BattensPlus.com This company sells a plastic looking batten for Class A tile roofs. This batten replaces wood battens. It will resist ponding since it has large fluted channels in it. It won’t absorb and hold moisture as wood does. It allows air circulation to dry the water that reaches under the tile, keeping the roof surface cool.
Ask the manufacturer for where to purchase and if this product can be used under other types of roofs. |
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Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
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jcwells
New Member
Posts:4
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| 17 Apr 2009 01:39 PM |
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I know this is moving away form the original topic but since we have discussed walls here goes.
The engineer asked me to consider the following product:
http://www.fifoil.com/Builders/Applications/Residential/ApplicationInfo/?ID=1
It is a radiant barrier designed exactly for the conditions I have which is firming strips on block. They claim an R value of 4.1 for the perforated hi perm version of the product in that application. Simple and probably cost effective.
Anyone had any experience with the product?
Thanks,
John
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Dana1
Senior Member
Posts:6991
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| 17 Apr 2009 02:51 PM |
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Radiant barriers are generally oversold (lot's of details in the fine print), but it WILL be useful for heat rejection with the ~1/2-3/4" air gap you have with the furring strips.
But it should be used in-addition to, not instead of EPS or fiber-faced iso sheathing on the inside though (and the perforated-breathable version is the right type to use here.) But it can let you get away with 1/2-3/4" instead of 1-1.5" rigid insulation (I prefer reinforced iso like Dow Sturdy-R over the more crumbly EPS with identical R-value per inch. It's semi-permeable @ ~3-perms. see http://files.buildsite.com/dbderived-f/dowchemical/derived_files/derived235874.pdf It may be a bit wimpy as an exterior sheathing, but it's easy to install as interior insulation without banging it up. 1/2" or 3/4" EPS is WAY too easy to damage, but it's wicked-cheap.)
Properly applied, radiant barriers do some good- improperly applied they're literally worthless. With the air gap between the radiant barrier and the hot surface, yes, something between 3-6 "R-value equivalent" would be it's anticipated performance. It can work in-contact with fiberglass too (fiberglass being somewhat translucent to radiated heat), but in most applications the air gap will be necessary (in some apps air-gaps on both sides will improve performance somewhat.) |
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dalyfrye
New Member
Posts:4
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| 28 Apr 2009 02:13 PM |
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I would be concerned about the roofing warranty when insulation is applied directly to the underside of the roof sheathing. This is considered to be a "hot" roof due to the lack of ventilation directly under the roof sheathing and that invalidates the warranty of most, if not all composition roofing materials, as that retained heat causes the roofing materials to deteriorate more rapidly than when roofing is applied to a "cold" roof system.
Other than this issue of warranty I would also be concerned about any leaks in the roof which might go undetected due to the seal provided by a closed cell poly. It would seem that damage could be much more extensive before becoming obvious. |
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Dana1
Senior Member
Posts:6991
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| 30 Apr 2009 08:27 AM |
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Posted By dalyfrye on 04/28/2009 2:13 PM
I would be concerned about the roofing warranty when insulation is applied directly to the underside of the roof sheathing. This is considered to be a "hot" roof due to the lack of ventilation directly under the roof sheathing and that invalidates the warranty of most, if not all composition roofing materials, as that retained heat causes the roofing materials to deteriorate more rapidly than when roofing is applied to a "cold" roof system.
Other than this issue of warranty I would also be concerned about any leaks in the roof which might go undetected due to the seal provided by a closed cell poly. It would seem that damage could be much more extensive before becoming obvious. The warranty issue is a non-starter. (Has ANYBODY ever collected on a shingle warranty, under ANY circumstances more than 5 years after installation?) The color of the roofing makes a bigger difference on peak & average material temperatures than whether it's insulated under the roof deck or not. The nailer-deck & roofing felt add up to an insulation value of about R1, so the exterior convective & radiant cooling will always provide more than 90% of the cooling, whether there's extra insulation value below or not. Many manufacturers, having looked at the issue, now warranty their product for hot-roof applications. (But even if they don't, don't sweat it.) Will it run hotter? Yes, maybe 5-10F hotter peak temps, 3-5F hotter average temps. There are bigger differences than that from being on the north vs. south pitch of the roof, or even slight regional differences. If that takes a year or so off the life of a 25 year shingle, who cares? You will have saved more than the cost of the replacement in heating/cooling energy costs. It's the decking, not the roofing, that gets cooled by roof ventilation, and decking generally more tolerant of the elevated temperatures. (You might see century or half-millenium differences in vented vs. non-vented decking problems, but I suspect most places will have other issues long before that's first-and-foremost, eh?) In hurricane and wildfire-prone areas it's been amply demonstrated that unvented roofs are less susceptible to high winds, and inhibits fire spread as well. In CA soffit & gable vents are a FIRE HAZARD, sucking burning embers into the attic during fire-driven wind events. In FL, vented attics allow wind driven upward pressures to develop under the roof deck, which in combination low pressure on the leeward slope creates significant mechanical loading in the opposite direction of the normal-gravitaional load of the roof (it can literally rip the roof off.) In new construction (or a major-reroofing) there's some energy and roofing longevity benefit to putting a secondary roof deck with a ventilation gap above the the primary roof deck. (Whether a dual-deck makes financial/economic sense from 5, 10, or 20 year present-value analysis isn't clear, but maybe, in some instances.) It's true that 2lb foam makes it more difficult to detect & find leaks. Not so for half-pound foam. If there's sufficient rafter depth to acheive the desired R-value, half-pound foam is also significantly cheaper per equivalent R-value, arguably a better value all the way around for that application. Florida Solar Energy Center at the University of Central Florida as done a lot of experemental study of the subject, and has compiled the condensed encylopedia of vented vs. unvented attic issues (from both their research as well as others) here: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
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Alton
Veteran Member
Posts:2164
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| 30 Apr 2009 11:14 AM |
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Dana1,
You covered the subject well. Very understandable and logical. I agree with your conclusions. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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sololoud
New Member
Posts:1
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| 02 May 2009 09:12 AM |
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If duct work is in the attic, use a product called solorguard by Guardian Products. This is attached to the roof joists in the attic and reduces the temperature in the attic. It is cheaper than any sprayform and does not mask any water leaks. It is safe product to use and performs as well, if not better than closed or open cell foam. It does not seal off the attic therfore you still have ventilation and you dont end up heating/cooling alot of extra conditioned space. Use traditional fiberglass blow insulation on the attic floor as normal. Most fiblerglass blown insulation now have the greenguard label, which makes it safe and green. Solorguard in not just a standard radiant barrier, check it out for yourself. I have talked with energy experts who have tested attics with infrared cameras and state the solorguard is just as good, if not better, safe and less expensive than any foam insulation sprayed in the roof. Remember open cell foam has a very high perm rating, which is a downfall in my opinion. Why condition a lot of extra place, think about it. I also understand the concept of conditioning the space for the duct work. But I dont buy it, you end up heating/cooling so much extra space, its not worth it. The concept behind the solorguard is it reduces the temperature in the attic, once this is done the R-6 or R-8 insulation on the ductwork does not have to work as hard to hold in the air temperature in the duct lines being supplied to the home. Therefore making the ductwork even more energy efficient. Check this out for yourself, it makes dollars and sense. Hope this helps. |
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Alton
Veteran Member
Posts:2164
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| 02 May 2009 09:46 AM |
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Sololoud,
Solorguard or Solarguard? I did not understand the part about this product (Solarguard) not masking water leaks. I also was confused with the implication that with sprayfoam the attic space required conditioning. I do not consider the attic conditioned unless a supply and return vent are installed. Most sprayfoamed attics that I have been in are not "conditioned". Help me out here. What am I missing in understanding Solarguard? |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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dmaceld
Veteran Member
Posts:1465
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| 02 May 2009 01:43 PM |
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Posted By sololoud on 05/02/2009 9:12 AM IWhy condition a lot of extra place, think about it. I also understand the concept of conditioning the space for the duct work. But I dont buy it, you end up heating/cooling so much extra space, its not worth it. Because the cost of heating/cooling a volume of air is not significant. Heat loss/gain in a house is almost entirely caused by transfer through walls, windows, doors, etc., and by air escape and infiltration. The number of BTUs of energy that it takes to heat the pounds of air in the attic space is almost nothing. Seal the attic tight and the heat/cool load for its volume is nothing to be concerned about. I don't have internet at home right now so I don't have the reference material at hand I need to give a reliable number for illustration.
My garage is insulated under the roof just like the house. The house attic is the return plenum for the heating system. It's the same temp and humidity as the living space. The garage attic is currently not vented to the garage space and I can tell it is not only warmer on a sunny day than the garage space, but more humid. I'm going to need to vent it to the garage space to keep it dry. |
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dmaceld
Veteran Member
Posts:1465
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| 02 May 2009 01:45 PM |
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Also, the Manual J calculation for my house showed no difference for heat loss/gain comparing under roof insulation with ceiling insulation even though the roof is 8/12 pitch.
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PanelCrafters
Advanced Member
Posts:680
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| 04 May 2009 07:12 AM |
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Posted By dmaceld on 05/02/2009 1:45 PM
Also, the Manual J calculation for my house showed no difference for heat loss/gain comparing under roof insulation with ceiling insulation even though the roof is 8/12 pitch.
Then I'd say that it wasn't done right!
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| ....jc<br>If you're not building with OSB SIPS(or ICF's), why are you building? |
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bmancanfly
New Member
Posts:79
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| 04 May 2009 07:49 AM |
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I keep going back and form on whether I think this spray foam is a good or bad idea. I think that the two greatest disadvantages are 1) cost. it's outrageously expensive. even with the tax credit it seems hard to workout a senario where i could recoup the cost in my lifetime. It's seems that a lot of the contractors in my area have responded to the tax stimulus plan by raising their price to absorb much of the tax credit for themselves. I'm getting bids averaging in the $8000 range for my basic 1600 sq ft ranch. 2) it's permenant. you can always remove fiberglass or cellulose and repalce it. once the foam is sprayed on, it's not coming off, ever, for any reason.
It feels like there might be some benefit based on the science, but the negatives seem to out weigh the cost. I mean if I use the Solarguard (or similar) on the underside of the roof decking, and load the ceiling with insulation it will still cost far less then the foam. Also, it can be a DIY project further reducing the cost - foam is not DIY. It's hard to imagine that the foam is going to provide that much more of an additional benefit to recoup the cost in any reasonable amount of time. |
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bmancanfly
New Member
Posts:79
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| 04 May 2009 08:42 AM |
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How do you apply the ISO board to the furring strips and the drywall to the ISO board? I wouldn't think that mechanical fasteners would work.
I only have 1 X 2 furring strips. |
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Dana1
Senior Member
Posts:6991
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| 04 May 2009 09:03 AM |
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Posted By PanelCrafters on 05/04/2009 7:12 AM [u]Posted By dmaceld on 05/02/2009 1:45 PM[/u]
Also, the Manual J calculation for my house showed no difference for heat loss/gain comparing under roof insulation with ceiling insulation even though the roof is 8/12 pitch.
[/quote]
Then I'd say that it wasn't done right!
It may/may not have been done right. The heat gain/loss in a well-sealed structure is all about the surface area & R whole-surface R value of the thermal envelope, which doesn't increase much. Extra volume is meaningless- it's all about the surface area. It could very well have been done right: The surface area of an 8/12 roof is only 20% more than the attic floor, and the rafter spacings forming thermal bridges in the insulation are usually wider than in an attic floor, so for equivalent center-cavity R-values (say R20 between the floor joist vs. R20 between the rafters) you end up with a difference of less than 15%, possibly less than 10%, well within the built-in margins of Manual J. Manual-J was never intended to be anything like a precision calculation, just a reliable way to get within +20% of reality in "typical" installations. With sprayed-roof/sealed venting, the fact that the ducts and air handlers etc are now inside the thermal & pressure envelope of building will more than make up for any theoretical efficiency losses from that 20% increase in surface area on the thermal boundary. (And if you give it 20% more center-cavity R-value to compensate, it'll be even better, eh?) And if you're comparing foam-insulated rafters to fiberglass-insulated floor (with no radiant barrier), at equivalent center-cavity R-values the difference in cooling load performance will be be HUGE! (Radiant gains will be cut by half.)
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PanelCrafters
Advanced Member
Posts:680
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| 04 May 2009 09:51 AM |
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Posted By Dana1 on 05/04/2009 9:03 AM
It may/may not have been done right. The heat gain/loss in a well-sealed structure is all about the surface area & R whole-surface R value of the thermal envelope, which doesn't increase much. Extra volume is meaningless- it's all about the surface area. It could very well have been done right:
The surface area of an 8/12 roof is only 20% more than the attic floor... Yup, but don't forget the walls! Most attics do have walls up there!
With sprayed-roof/sealed venting, the fact that the ducts and air handlers etc are now inside the thermal & pressure envelope of building will more than make up for any theoretical efficiency losses from that 20% increase in surface area on the thermal boundary. I hope that this isn't a surprise, but in many areas of the country, there is no ductwork or mechanical systems within the attic space. In fact I've never lived in a place where there was.  |
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| ....jc<br>If you're not building with OSB SIPS(or ICF's), why are you building? |
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Dana1
Senior Member
Posts:6991
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| 04 May 2009 10:15 AM |
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Most attics in FL don't have walls, and many/most post-1970 construction in FL has at least ductwork up there, if not the air-handler too.
I'd hazard that most slab-on-grade homes in the lower-48 with central-AC &/or hot-air heating have at least some ductwork in the attic- it isn't rare.
In New England attics that have walls are called "spare bedrooms" ( ;-) )...
...and should rightfully be brought inside the thermal & pressure boundaries of the building envelope, even if only as semi-conditioned space without zoned temperature control. (Leave any existing attic-floor insulation in place- it's performance will be enhanced by the sealed attic ventilation, working better than it ever has, since the infiltration factor will have dropped an order of magnitude, and the delta-T it's subjected to is halved.)
Unless you're using gasketed hatches and spray foam on the floor it's very difficult to control the infiltration factor between the upper floor & attic well. Between plumbing & electrical chases, flues & partition-wall cavities (not to mention the ubiquitous recessed light fixtures), sealing the attic floor can be a labor-intensive and painstaking operation to do right as a retrofit. Rafters are easy by comparison. |
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dmaceld
Veteran Member
Posts:1465
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| 05 May 2009 08:58 PM |
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Posted By PanelCrafters on 05/04/2009 9:51 AM [u]Posted By Dana1 on 05/04/2009 9:03 AM[/u] It may/may not have been done right. The heat gain/loss in a well-sealed structure is all about the surface area & R whole-surface R value of the thermal envelope, which doesn't increase much. Extra volume is meaningless- it's all about the surface area. It could very well have been done right: The surface area of an 8/12 roof is only 20% more than the attic floor...[/quote] Yup, but don't forget the walls! Most attics do have walls up there!
With sprayed-roof/sealed venting, the fact that the ducts and air handlers etc are now inside the thermal & pressure envelope of building will more than make up for any theoretical efficiency losses from that 20% increase in surface area on the thermal boundary. [/quote] I hope that this isn't a surprise, but in many areas of the country, there is no ductwork or mechanical systems within the attic space. In fact I've never lived in a place where there was. Dana1, thanks for backing me up! Panel, it would be more correct to say 'very little' or 'insignificant' difference rather than 'no difference'! I actually was a bit surprised when the results came out that way.
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